Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Also, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation leads to stimulate the tissue and provide the desired efficacious therapy to the patient. The neurostimulation system may further comprise a handheld patient programmer in the form of a remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a clinician, for example, by using a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
In the context of an SCS procedure, one or more stimulation leads are introduced through the patient's back into the epidural space, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. One type of commercially available stimulation leads is a percutaneous lead, which comprises a cylindrical body with ring electrodes, and can be introduced into contact with the affected spinal tissue through a Touhy-like needle, which passes through the skin, between the desired vertebrae, and into the epidural space above the dura layer. For unilateral pain, a percutaneous lead is placed on the corresponding lateral side of the spinal cord. For bilateral pain, a percutaneous lead is placed down the midline of the spinal cord, or two or more percutaneous leads are placed down the respective sides of the midline of the spinal cord, and if a third lead is used, down the midline of the special cord. After proper placement of the stimulation leads at the target area of the spinal cord, the leads are anchored in place at an exit site to prevent movement of the stimulation leads. To facilitate the location of the neurostimulator away from the exit point of the stimulation leads, lead extensions are sometimes used.
The stimulation leads, or the lead extensions, are then connected to the IPG, which can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted tissue, and in particular, the dorsal column and dorsal root fibers within the spinal cord. The stimulation creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. Intra-operatively (i.e., during the surgical procedure), the neurostimulator may be operated to test the effect of stimulation and adjust the parameters of the stimulation for optimal pain relief. The patient may provide verbal feedback regarding the presence of paresthesia over the pain area, and based on this feedback, the lead positions may be adjusted and re-anchored if necessary. A computer program, such as Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation, can be incorporated in a clinician's programmer (CP) (briefly discussed above) to facilitate selection of the stimulation parameters. Any incisions are then closed to fully implant the system. Post-operatively (i.e., after the surgical procedure has been completed), a clinician can adjust the stimulation parameters using the computerized programming system to re-optimize the therapy.
Multi-lead configurations have been increasingly used in electrical stimulation applications (e.g., neurostimulation, cardiac resynchronization therapy, etc.). In the neurostimulation application of SCS, the use of multiple leads increases the stimulation area and penetration depth (therefore coverage), as well as enables more combinations of anodic and cathodic electrodes for stimulation, such as transverse multipolar (bipolar, tripolar, or quadra-polar) stimulation, in addition to any longitudinal single lead configuration.
Several studies have demonstrated the advantage of using narrowly spaced, parallel leads placed symmetrically on both sides of the physiological midline in improving penetration and paresthesia coverage (see J. J. Struijk and J. Holsheimer, Tripolar Spinal Cord Stimulation: Theoretical Performance of a Dual Channel System, Medical and Biological Engineering and Computing, Vol. 34, No. 4, 1996, pp. 273-279; J. Holsheimer, B. Nuttin, G. King, W. Wesselink, J. Gybels, and P. de Sutter, Clinical Evaluation of Paresthesia Steering with a New System for Spinal Cord Stimulation, Neurosurgery, Vol. 42, No. 3, 1998, pp. 541-549; Holsheimer J., Wesselink, W. A., Optimum Electrode Geometry for Spinal Cord Stimulation: the Narrow Bipole and Tripole, Medical and Biological Engineering and Computing, Vol. 35, 1997, pp. 493-497).
The efficacy of SCS is related to the ability to stimulate the spinal cord tissue corresponding to evoked paresthesia in the region of the body where the patient experiences pain. Thus, the working clinical paradigm is that achievement of an effective result from SCS depends on the neurostimulation lead or leads being placed in a location (both longitudinal and lateral) relative to the spinal tissue such that the electrical stimulation will induce paresthesia located in approximately the same place in the patient's body as the pain (i.e., the target of treatment). If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy.
When lead migration occurs, proper paresthesia coverage can most often be recaptured by reprogramming the IPG, e.g., by using the Bionic Navigator® software. When multiple percutaneous leads are used, programming of the IPG often requires the knowledge of the relative positions between the leads in order to properly place the poles of the generated electrical field. Such information, however, is not readily available to the programmer unless fluoroscopic imaging is performed. Fluoroscopic imaging involves ionized radiation, adds time and cost, and requires a bulky instrument, both of which may limit its use in the clinical setting, and effectively prevent its use outside of the clinical setting. If such lead placement is not detected in a timely manner before the system is fully implanted, it may result in inefficient therapy and possibly require a second surgery for lead revision. Thus, lead migration continues to be the most common technical complication of spinal cord stimulation therapy. A literature review suggested the incident rate of lead migration was approximately 13.2% (see T. Cameron, Safety and Efficacy of Spinal Cord Stimulation for the Treatment of Chronic Pain: a 20-Year Literature Review, J Neurosurg: Spine 2004, 100: 254-267).
Currently, the relative positions of multiple leads can be electronically estimated by measuring electrical signals between lead electrodes. For example, the Bionic Navigator® software uses an Electronically Generated Lead (EGL) Scan that estimates the relative lead positions by examining the profiles of electrical field potential measured from the electrodes that is generated due to the current flow in the medium. In particular, the EGL scan feature detects the stagger of the leads by comparing the profile of measured cross-lead field potentials with those calculated from a Finite Element Model (FEM) for known lead configurations. The FEM model provides a prediction of the field potentials that are expected to be measured on the electrodes, and it takes into account the geometric properties and electrical behaviors of the various elements in the spinal cord, as well as the boundary conditions imposed on the field potentials generated in the human body. The lead stagger can be determined by comparing the profile of the measured cross-lead field potentials with those computed from the FEM model for several known lead configurations. The lead configuration for which the modeled field potential profile best matches the measured field potential profile is designated as the detected lead configuration.
This technology provides an automated means to obtain the information about the relative position of implanted leads without using fluoroscopy. Such information may be used to increase programming accuracy, thus improving the patient outcomes and treatment efficacy. Although the comparison of the measured field potential profile to numerous reference profiles generated by the FEM model is computationally intensive and requires a lot of memory, the CP is embodied in a computer with the processing power and memory necessary to efficiently perform these computations.
As the next generation SCS systems are expected to give the patient more control over their stimulation programs to improve the therapy, as well as to reduce the need for office visits, it has been proposed to incorporate more programming features (previously reserved for CPs) into RCs and IPGs. Just as in the programming of the IPG through a CP, it is also desirable to have the capability of electronically determining the relative lead positions within the RC, which may be needed to properly program the IPG. However, in the case of the EGL Scan feature, the known field potential profiles to which the measured cross-lead field potentials are compared to determine the lead stagger is stored in a database that is loaded during the EGL Scan processing. This presents a potential difficulty in transferring the present EGL Scan features directly into a RC, because it requires memory space for the database storage that may not be available in the RC.
There, thus, remains a need for a technique that determines the relative positions of leads without requiring a large amount of memory space.